The art of diagnostic imaging exploits compounds that in binding or localizing site selectively within the body, help to resolve the image of diagnostic interest. Diagnostic imaging agents are effective for targeting and imaging biological receptors. These diagnostic imaging agents incorporate radionuclide metals such as technetium and rhenium. The radionuclide metals are used to label targeting molecules, such as proteins, peptides and antibodies that localize at desired regions of the human body. Localization of these agents is detected by gamma camera analysis. As targeting agents, proteins and peptides can offer the tissue specificity required for diagnostic accuracy.
Labeling of these and other diagnostic agents with metal atoms is made difficult by their chemical structure. Conventional labeling techniques involve the formation of the metal complex in a solution of excess ligand which typically results in high levels of unlabelled ligand. For example, technetium labeling reactions yield approximately one labeled ligand for every thousand or more unlabeled ligand. For many radiodiagnostic agents there is a finite number of binding sites ie. receptors for which both the labeled and unlabeled diagnostic agent compete. This leads to poor imaging because the excess unlabeled ligands compete with the labeled ligands for the binding sites. As a result, the administration of larger doses of the agent is required to achieve an acceptable image. The administration of larger doses of the agent into a patient can lead to adverse effects. Targeting molecules often cause physiological changes when they bind or localize to specific sites. For example, the targeting molecule may be a peptide that binds to a receptor and agonizes or antagonizes receptor activity. Unlabeled peptide will still cause the physiological effect of agonizing or antagonizing receptor activity, however, the benefits of a metal labeled peptide will not be obtained.
Presently, high performance liquid chromatography (HPLC) is applied to enhance the concentration of labeled agent in the solution before it is administered. While enhancing concentration, this separation step requires additional time and expense. This makes the technique impractical for clinical use.
A method for labeling biological targeting molecules with radionuclide metals is described in U.S. Pat. No. 5,789,555. The method involves the steps of covalently linking a chelating portion of a biological targeting molecule to a solid support through a metal cleavable maleimide linker. When a radionuclide metal is introduced, it complexes with the chelating portion of the biological targeting molecule. This results in the release of the biological targeting molecule from the support yielding a labeled biological targeting molecule. This method provides a high ratio of labeled to unlabeled molecules (specific activity). However the use of the linker requires an extra step of coupling the linker to the support. The requirement of a linker is therefore a drawback of this method.
Many labeling methods require the formation of the free thiol. The thiols tend react with each other forming disulphides. Extra precautions need to be taken to avoid disulphide formation which results in the reduction of product attached to the surface and the need for extra purification before radiolabeling.
Many metal chelators contain a sulfur atom which participates in the chelating function. Sulfur atoms are commonly rendered unreactive by the use of protecting groups, one of which is acetamidomethyl. The sulfur is protected with an acetamidomethyl group. The sulfur protecting groups must be removed to provide a free thiol. The acetamidomethyl protecting group is removed from the sulfur through the use of mercuric acetate. Mercuric acetate coordinates to the sulfur which then possesses a slightly positive charge making the chelator a good leaving group. The excess mercuric acetate is removed by reaction with hydrogen sulphide. Mercuric acetate is quite toxic. Therefore the requirement to deprotect sulfur with mercuric acetate is a drawback.
The labeling method described in U.S. Pat. No. 5,789,555 makes use of organic solid supports that consist of a plurality of particles. The particles can be difficult to filter for the subsequent removal of the unbound and labeled biological targeting molecules. These supports are very susceptible to radiolytic degradation due the emission of Particles by certain radionuclide metals. These supports are also otherwise not chemically robust. This makes the solid support difficult to sterilize since sterilization is carried out at very high temperatures.
There is therefore a need for a solid support for labeling biological targeting molecules with a radionuclide metal that permits labeling be carried out in a single step without the need for linker group. There is a need for a solid support for labeling biological targeting molecules with a radionuclide metal that has increased chemical robustness, reduced radiolytic degradation, is easily sterilized, and labeled and has well defined loading characteristics. There is further need for such a solid support that can directly bind a protected thiol group to form a bound thiol thereby avoiding the formation of disulfides.
There is a further need for an improved system that employs a surface as a substitute for mercury so that no mercuric acetate needs to be used or subsequently removed in order to deprotect sulfur atoms. There is a need for such a system that eliminates a few synthetic steps as well as the presence of mercury in a reaction solution.